This. When going 50km/h on a tall custom map or Côte d'Azur in GT5 it feels like going 20km/h in real life. Can't say about 150+ km/h like people do when racing since I never drove those speeds on public roads except the Autobahn, what I can say for sure though is that even with cars around you on Route X 350km/h feels barely like you're traveling a totally awesome speed. This may be down to lack of gravity in a video game but there are graphical effects (that NFS Shift uses for example) which may not be realistic but give you a feeling that is closer to reality.
I think GT5 tries too hard to be objective in order to look 'realistic', GT6 should concentrate more on your subjective feelings while driving than pretending to use effects that do not exist in reality.
After all, lens flare is something that makes GT5 looking more realistic while it is only a way to replicate what you see in reality and not something that actually exists.
Because the ultimate driving simulation is not only about racing. It's also about normal city driving, especially on the city courses that follow actual streets (Monaco being a very good example in GT5). It's fun, especially with lots of friends doing the same and making the road two way.